1. Field of the Invention
The present invention relates to a magnification optical system which can be used in various kinds of projector.
2. Description of the Related Art
Referring to FIG. 1, the structure and function of a general projector type image display device using a reflective light bulb is described.
In FIG. 1 a projector includes a reflective light bulb LB, a light source LS, a mirror M, an integrator rod IR, a lens LN, a curved mirror CM, and a magnification optical system POS.
The light source LS includes a lamp LP and a reflector RF to project a light beam to the light bulb LB.
The integrator rod IR, lens LN, mirror M, and curved mirror CM constitute an illumination system to guide the light beam from the light source LS to the light bulb LB.
The integrator rod IR is a light pipe made of four mirrors combined as a tunnel, to reflect an incident light beam with mirror surfaces to an exit.
The magnification optical system POS projects the reflected beam from the light bulb LB onto a target surface or a screen to form an enlarged image thereon. The light bulb LB is a digital micro mirror device (DMD) in which micro mirrors are arranged in array. The normal line of the micro mirrors can be changed independently from each other by ±12 degrees, for example.
The light from the lamp LP is reflected by the reflector RF, converged on the entrance of the integrator rod IR, repeatedly reflected therein, and projected as a light beam with uniform luminance. Then, the light beam illuminates the light bulb LB via the illumination system.
The illumination system converts the light beam from the integrator rod IR to a surface light source with uniform luminance and forms an image of the surface light source on the light bulb LB.
The positions of the light bulb LB and the magnification optical system POS are determined so that light is reflected by the micro mirrors in the light bulb LB to be incident on the magnification optical system POS when the micro mirrors are inclined by −12 degrees and light reflected thereby is not incident on the magnification optical system POS when the micro mirrors are inclined by +12 degrees. Then, the direction in which the light beam from the curved mirror CM is incident on the light bulb LB is decided.
An image can be displayed on the light bulb LB by adjusting the inclination of each micro mirror in accordance with the pixels of an image projected on a target surface.
By illuminating the light bulb LB on which the image is displayed with light, the light beam reflected by each micro mirror is incident on the magnification optical system and converted thereby to imaging light. The imaging light forms an enlarged image of the image on the light bulb Lb on the target surface. This image is called as projected image.
Since the light bulb is illuminated with light with uniform luminance distribution, the projected image has uniform illumination distribution. Thus, a digital image is displayed on the target surface.
The projector functions to project an image as a real image of the image on the light bulb LB onto the target surface such as a screen. The size of the projected image or the distance from the projector to the target surface differs depending on the specific condition of the projector in use.
A projected image needs to be brought into focus on the target surface. FIG. 2A shows a magnification optical system POS1 including lenses in coaxial, rotational symmetry with an optical axis AX. The focus of an image on the screen SC is generally adjusted by moving the entire magnification optical system or moving a focus lens group.
FIG. 2B shows a projector which uses a magnification optical system comprising a refracting optical train POSL1 and a mirror train POSM1 (a single concave mirror in the drawing) not coaxial with the refracting optical train POSL1. It aims to project images in a closer distance than a related art projector.
To be easily viewable in an extremely close distance, an image needs to be projected above the projector. The light bulb LM (DMD) is eccentrically disposed with its center off the optical axis AX of the magnification optical system as shown in FIG. 2A, 2B. To realize a wide projection area and maintain image quality, a wide angle lens is used for the magnification optical system. However, there is a limitation to widening the angle of the magnification optical system of lenses in coaxial, rotational symmetry, and an optical path has to be extended by a mirror train in order to project images from an extremely close position adjacent to the screen SC.
An image can be projected in a close distance by oblique projection by which the optical path of the magnification optical system is reflected by a planar mirror to incline the optical axis thereof relative to the screen. However, this type of projection faces a problem that a projected image is distorted to a trapezoidal shape.
In FIG. 2B the concave mirror of the mirror train POSM1 with a free-form curved surface can effectively correct a trapezoidal distortion in a projected image. The correction of trapezoidal distortion with a free-form curved mirror is disclosed in detail in “Optical and Electro-optical Engineering Contact, Vol. 39, No. 9 in 2001 by Japan Optomechatronics Association”.
A floating focus system is suitable for the projector comprising the refracting optical train POSL1 and the mirror train POSM1 including a free-form curved surface to correct a trapezoidal distortion in FIG. 2B. This system is to perform focusing at an extremely close range by fixing one or more lenses closest to the light bulb LB and moving the other lens groups and mirrors along the optical axis like moving “floating trees”. It is widely applied for an interchangeable lens of a single lens reflex camera.
However, it is not possible to sufficiently correct the trapezoidal distortion in an image projected at an extremely close distance by focusing with a single lens or lens groups or protruding the entire magnification optical system. Further, curvature of field cannot be sufficiently corrected, leading to blurs in the center and periphery of the display.
Meanwhile, the floating focus system can properly correct trapezoidal distortion and curvature of field in an image projected from an extremely close distance by the non-coaxial curved mirror.
This is described in detail referring to FIGS. 3A, 3B. FIG. 3A shows a projector which obliquely projects images. A magnification optical system POS0 projects an image on the screen SC. A planar mirror to reflect an optical path is omitted therefrom for simplicity.
The display surface of the light bulb LB as DMD is of a rectangular shape with vertical (Y direction) short sides but the projected image is a trapezoidal shape as shown in FIG. 3B.
FIG. 4A shows another projector comprising a magnification optical system having a refracting optical train POSL1 and a mirror train POSM1 as a concave mirror. The surface of the light bulb LB on which images are displayed is rectangular as shown in FIG. 4B. The refracting optical train POSL1 forms a real image on the display as an intermediate image Im0 between the refracting optical train POSL1 and the mirror train POSM1. The mirror train POSM1 projects the intermediate image Im0 on the screen SC as an object image.
The intermediate image Im0 formed by the refracting optical train POSL1 is distorted to a trapezoid with a narrow top portion as shown in FIG. 4B. The distortion is corrected by the mirror train POSM1 and a corrected rectangular image is projected on the screen SC, as shown in FIG. 4B.
To project a smaller image onto the screen SC with the projector in FIG. 4A, the screen SC is moved rightward in Z direction from the position in FIG. 4A and focus adjustment is performed by protruding the coaxial refracting optical train POSL1 in Z direction as shown in FIG. 5A.
The distortion in the intermediate image Im0 shows almost no change before and after the protrusion of the refracting optical train POSL1 and the shape thereof in FIG. 5B is similar to that of the screen in FIG. 4B. Accordingly, a trapezoidal distorted image in FIG. 5B is projected on the screen SC.
This effect is described in detail with reference to FIGS. 6A to 6D. FIG. 6A shows an X to Z cross section of FIG. 4A. In FIG. 6A the magnification optical system including the concave mirror POSM1 projects light at different angles upward and downward in Y direction on an XZ cross section of the screen SC. FIG. 6C shows a rectangular image properly projected by the magnification optical system in FIG. 6A.
When the screen SC is moved as in FIG. 5A, a trapezoidal distortion with a short top side occurs due to light's reaching different positions in X direction on top and bottom of the screen SC, as shown in FIG. 6B.
When the light bulb and the refractive optical system are disposed in a proper distance along the normal line of the light bulb, the floating focusing is very effective to correct trapezoidal distortion in an image and curvature of field. Further, owing to the good correction of curvature of field, the floating focusing is effective when focus adjustment amounts are largely different in the top and bottom of the display, for example, when the screen SC is moved to the curved mirror POSM1 from a position SC1 (FIG. 4A) to a position SC2 (FIG. 5A).
Meanwhile, for correcting the same focus adjustment amount on the entire screen, not the floating focusing but the focusing by protruding the entire magnification optical system or the front lens group is effective.
Various methods for the focusing of the projector are well known, for example, disclosed in Japanese Patent Application Publication No. 2009-251457, No. 2009-229738, and No. 2008-165187.
Thus, floating focusing can correct blurs in the center and periphery of an image on the display but it cannot deal with blurs in the entire image due to a variation in the distance between the refractive optical system and the light bulb or a variation in the focal length of the refractive optical system.